Add __VA_OPT__

preprocess.c

@@ -503,6 +503,13 @@ static Token *paste(Token *lhs, Token *rhs) {
   return tok;
 }
 
+static bool has_varargs(MacroArg *args) {
+  for (MacroArg *ap = args; ap; ap = ap->next)
+    if (!strcmp(ap->name, "__VA_ARGS__"))
+      return ap->tok->kind != TK_EOF;
+  return false;
+}
+
 // Replace func-like macro parameters with given arguments.
 static Token *subst(Token *tok, MacroArg *args) {
   Token head = {};
@@ -565,6 +572,17 @@ static Token *subst(Token *tok, MacroArg *args) {
       continue;
     }
 
+    // If __VA_ARG__ is empty, __VA_OPT__(x) is expanded to the
+    // empty token list. Otherwise, __VA_OPT__(x) is expanded to x.
+    if (equal(tok, "__VA_OPT__") && equal(tok->next, "(")) {
+      MacroArg *arg = read_macro_arg_one(&tok, tok->next->next, true);
+      if (has_varargs(args))
+        for (Token *t = arg->tok; t->kind != TK_EOF; t = t->next)
+          cur = cur->next = t;
+      tok = skip(tok, ")");
+      continue;
+    }
+
     // Handle a macro token. Macro arguments are completely macro-expanded
     // before they are substituted into a macro body.
     if (arg) {

test/macro.c

@@ -377,6 +377,11 @@ int main() {
 
   ASSERT(0, strcmp(__BASE_FILE__, "test/macro.c"));
 
+#define M30(buf, fmt, ...) sprintf(buf, fmt __VA_OPT__(,) __VA_ARGS__)
+  ASSERT(0, ({ char buf[100]; M30(buf, "foo"); strcmp(buf, "foo"); }));
+  ASSERT(0, ({ char buf[100]; M30(buf, "foo%d", 3); strcmp(buf, "foo3"); }));
+  ASSERT(0, ({ char buf[100]; M30(buf, "foo%d%d", 3, 5); strcmp(buf, "foo35"); }));
+
   printf("OK\n");
   return 0;
 }